Gas meters, particularly for industrial use, are required to measure large volumes of gas for heating or cooling purposes. Most gases are normally sold to the industrial user on the basis of price per volume. Natural gas is a relatively expensive commodity and when used at high volume rates it is necessary to accurately measure the amount of gas being consumed. Accurate measurement prevents the consumer from being overcharged by the provider and it also ensures that the consumer is charged for the gas provided.
A common method of providing accurate measurement of consumed gas is the use of positive displacement rotary gas meters, specifically those that are electronically compensated. When gas flows through such rotary gas meters, fixed volumes of gas are displaced by, for example, two figure-eight impellers that rotate in opposite directions within a cylinder of known volume. The impellers of the meter rotate because of a lower differential pressure at the outlet of the meter than is present at the inlet. As they rotate, a fixed volume of gas or other fluid is entrapped and then moved toward the outlet. Therefore, with each full rotation a known volume of gas or other fluid is displaced through the outlet.
By measuring the number of rotations of the impellers, the volume of gas or other fluid displaced over a period of time can be determined. And because the lobed figure-eight impellers remain in a fixed relative position, it is only necessary to measure the rotational movement of one of them. To accomplish this, normally, the impeller is magnetically coupled to an electronic recording device or geared to a mechanical register.
Typically, a magnetic coupling device senses movement of the impellers by sensing the passage of magnets fixed to the rotating impellers. This is done with a Wiegand sensor mounted outside the pressure body of the gas meter. The sensor then transfers a signal to the electronic recording device. This device compensates for density changes due to temperature fluctuations in the gas being metered, resulting in an extremely accurate measurement of the consumed gas.
However, electronically compensated gas meters can fail to record the amount of volume passing through it for a variety of reasons. The most common failures are due to power loss, faulty electronic components, and failures due to lightning or vandalism. If the module assembly stops recording for any reason, there is a need for a device to account for the flow. Billions of dollars are lost each year by gas providers over disputes with consumers as a result of failures of electronically compensated gas meters.
A mechanical register can conveniently account for the flow in the event of a failure of an electronically compensated gas meter. Typically, such mechanical registers display the unconverted volume of consumed gas. This displayed volume can be used to determine the amount of gas that has flowed through the pressure body and acts as a backup to the electronic recording device.
The mechanical register uses a complicated gear assembly that is mechanically actuated by the rotation of the impellers of the gas meter and which in turn drives a series of gear combinations in response to the displacement of gas through the meter which in turn drives the uncorrected volume register, as it is conventionally known. That is, gas flowing through the meter causes rotation of the impeller of the gas meter, which actuates a gear drive assembly to measure actual volume of gas flowing through the meter without regard to the temperature.
However, such complicated mechanical registers limit the performance of gas meters by affecting the measurement of low gas flow rates. By using a complicated gearbox, torque from the gears interferes with the impellers by creating a pressure drop between the two sides of the impellers. This, in turn, impairs the ability to measure low flow rates because, at low flow, the torque from the gears significantly affects the normal rotation of the impellers.
It is desirable to minimize the pressure drop that is caused by the mechanical register, thereby permitting the impellers to rotate in a normal manner, especially to maintain the ability to accurately measure low gas flow rates. Accordingly, there is a need for an improved mechanical backup measuring device, which allows for the consumer or gas provider to resolve disputes over the amount of gas used without affecting the measurement of low gas flow rates.